1. Field of the Invention
The present invention relates to a device for transporting web-shaped recording substrates in printing devices including a first and a second transport device, wherein the transport devices are in each case driven by a gear at different speeds, and wherein one of the transport devices is in addition driven by a coupling.
2. Brief Description of the Background of the Invention Including Prior Art
Modern printing devices, for example ink jet printers, are characterized by a marked user-friendly operating procedure during the printing of web-shaped recording substrates. The operational friendliness of a printing device is of particular importance for the user, where the printing device is to print on sheet-shaped recording substrates in addition to web-shaped recording substrates. In order to fulfill this requirement, however, more complicated, mechanical transport devices are necessary with which the sheet-shaped or, respectively, web-shaped recording substrates are fed to a printing station. In the following, the term continuous form paper or, respectively, individual sheet is employed for the term web-shaped or, respectively, sheet-shaped recording substrate. A recording substrate is intended in this case to include recording substrates which can differ both in their kind, for example, paper, cardboard, foil, as well as in the grade of pretreatment, for example, edge perforation, preprinted forth.
If for example a user wishes to print a single sheet after the printing of alphanumerical characters onto the continuous form paper, then the unprinted part of the continuous form paper, still disposed in a roller wedge of the transport device, has to be moved into a stand-by position for the next printing procedure. The continuous form paper, disposed in the stand-by position, has to be fed again to the printing station following the printing of the individual sheet.
A further requirement associated with such a transport device is brought about in that a printing procedure, interrupted for a certain time period (printing pause), results in that the continuous paper is moved to a so-called tear-off position. If the printing procedure of the continuous form paper is now to continue from the preceding printing procedure, then the continuous form paper has to be transported anew from the tear-off position back into the printing position.
A paper transport device for individual sheets and continuous form paper in printing devices is known from the European Patent document EP-A2-0,123,310. The paper transport device exhibits for this purpose a switching gear, which mechanically couples a drive motor to a printer platen or, respectively, the drive motor to a paper tractor and the printer platen dependent on the operation type, single sheet or continuous form paper, of the printing device. The switching gear is in this case formed such that no malfunctions, such as for example paper jamming, occur during the transition from the operation type continuous form paper to the operation type single sheet or vice versa. The back transport of the individual sheets or, respectively, of the continuous form paper is achieved by the rotation direction reversal of the drive motor. It is a disadvantage of the known paper transport device that the mechanical coupling between the paper tractor and the printer platen has to be very accurate in order to be able to transport the edge-perforated continuous form paper without any problems, perfectly, and accurately in its position.
Furthermore, a paper feed device for printing devices is known from the U.S. Pat. No. 4,688,957, wherewith individual sheets or continuous form paper can selectively be fed over a friction roller to a printing station. A paper tractor, separating rollers, and the friction roller are in this case driven over a gear by an electromotor. A gear coupling is provided in the gear for the selection of the operation type single sheet or continuous paper, where the gear coupling can be manually operated with an operating lever. Since the paper tractor for both transport directions of the continuous form paper is at all times coupled to the friction roller in case of the known feed device during the continuous form paper operation, the same disadvantages occur as with the technical teachings of the European Patent document EP-A2-0,123,310.
In addition it is possible to transport continuous form paper, for example edge-perforated fanfold paper, only backwards, jointly over a tractor and a printer platen. In this case, both the paper tractor as well as the printer platen are driven by a step motor, which is in each case connected over a gear with the tractor and the printer platen. A freewheel is disposed in the gear between the paper tractor and the step motor, where the freewheel blocks in reverse direction. In forward direction, the edge-perforated fanfold paper is manually shifted by rotation of a handwheel from an insertion or, respectively, stand-by position in a roller wedge of the printer platen. In this case, the handwheel of the paper tractor is to be rotated in a direction opposite to the handwheel of the printer platen. When the edge-perforated fanfold paper reaches the roller wedge, then the friction drive of the printer platen takes over the further transport. The advance rate of the edge-perforated fanfold paper is exclusively determined by the friction drive of the printer platen. The friction drive is thereby disengaged from the paper tractor in forward direction by the effective freewheel. This is achieved in that the freewheel is driven faster by the step motor than the edge-perforated fanfold paper is carried along by the friction drive. During the pulling in of the edge-perforated fanfold paper, the paper tractor is pulled along by the edge-perforated fanfold paper.
This principle of pulling in and back transportation of edge-perforated fanfold paper has the disadvantage that the edge-perforated fanfold paper, disposed in the stand-by position, can only be moved slightly, i.e. a maximum of 1/6 inch, backward through the printer platen. If the edge-perforated fanfold paper is moved backwards by a larger distance, then the paper tractor pushes the edge-perforated fanfold paper backwardly out of the printer device as a result of the blocked freewheel. This occurs for example during the change from the continuous form paper operation to the single sheet operation in a printing device. Such a backward motion occurs however for example during the printing of several exponents onto a single sheet.